Problem: In physics class, the girl to boy ratio is $2$ to $10$. If there are $10$ girls, how many boys are there?
Solution: A ratio of $2$ girls to $10$ boys means that there is a group of $2$ girls for every group of $10$ boys. If there are $10$ girls, then there are $5$ groups of $2$ girls each. Because we know the ratio is $2:10$ , there must be $5$ groups of $10$ boys each. There is a total of $50$ boys in physics class.